The present invention relates to electrical switching devices and more particularly to controlling the operating pressure of the device to minimize contact damage and erosion during the separation of the contacts.
The invention is applicable to a wide variety of electrical switches ranging from low voltage a.c. switch-contactors or circuit-breakers, which operate at from about 110 to 440 volts a.c., to highly specialized low d.c. voltage shunt bypass switches for electrochemical cells.
In the low voltage a.c. type switches used for distribution and motor control, the preferred contact material is an alloy containing significant proportions of silver, which is a high cost noble metal. These type of switches are typically open to ambient atmospheric pressure air. Silver is used as the contact because silver forms a conducting oxide which maintains high electrical conductivity and efficient current flow through the closed switch contacts. Recent shortages of materials such as silver have dramatically increased the cost of such contacts, and it is obviously desirable to eliminate or minimize the need for high cost, noble metal contact constituents.
In low voltage d.c. shunt bypass switches for electrochemical cells, which are operated at typically less than 10 volts d.c. with continuous current ratings of about 6,000 amperes for a single switch, the switch is typically a hermetically sealed, high vacuum device. The contacts are typically copper or copper-bismuth, high conductivity material, with requisite weld-break characteristic upon switch opening. The oxide of copper is a poor conductor and the vacuum condition within the switch prevents oxidation of the copper contacts.
When switch contacts are moved apart to interrupt the current flow an arc forms between the parted contacts with localized heating and erosion occurring till the arc is extinguished.
The present inventor had earlier observed and reported that the vacuum-arc cathode erosion rate can be decreased by more than an order of magnitude with increasing ambient pressure, as reported in "Cathode Spot Erosion And Ionization Phenomena In Transition From Vacuum To Atmospheric Pressure Arcs" Journal of Applied Physics, Volume 45, Number 12, pp. 5235-5244 (1974), and in "Anode Phenomena In Vacuum And Atmospheric Pressure Arcs", Volume PS-2, pp. 310-319, December 1974.
In an early work by others, "Arc Cathodes of Low Current Density at High Amperage", Journal of Applied Physics, Vol. 13, February 1942, p. 113-116, it was reported that arcs with low current density cathodes could be formed in low density gas to reduce electrode surface destruction.